Method and device for mapping radiation sources

ABSTRACT

A method and apparatus for precisely locating radioactive sources. A pair of visual cameras are oriented in directions so that they have all or part of their field of vision in common. The apparatus also includes an intermediate camera which is sensitive to the radiation to be measured. The visual cameras make it possible to define the position of the details of the environment by a triangulation method and another triangulation is carried out in order to know the position of the sources after having moved the apparatus. Photogrammetry software is used to accomplish these tasks easily.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a method and device for mapping radiationsources, to make it possible to locate such sources in athree-dimensional environment which may be known or unknown at thestart.

2. Discussion of the Background

The idea of detecting radiation sources such as radioactive leaks bycomparing an image of these sources, taken by a specialised item ofequipment, and a visual or video image of the environment taken by anordinary light-sensitive camera has already been applied. The use of theimages can be effected empirically by the user, finding the locations ofthe radioactive sources on the visual image; in a rudimentary design ofthe method, he identifies the elements of the environment, restored onthe visual image, which correspond to the places on the image of thesources on which a source has been recorded. The identification of theposition of the sources is however not very precise and calls on thejudgement and knowledge of the user with regard to the environment: hemay thus know in advance the places in the environment where a leakageis liable to occur and identify them easily when it arises. Though sucha method is sufficient in certain situations, it does not lend itself toautomatic processing, notably if repair work is to be entrusted to arobot, which must know precisely the position of the source inthree-dimensional space in order to reach it.

In a more improved method which is described in French patent No 2 652909, a camera is used which is combined with obturators and means ofconverting the radioactive radiation, which enables it to record in turna visual image and a radioactive emission image of the environment. Thetwo images are supplied to automatic processing means, which superimposethem, which gives, without error, the position of the sources in thefield of the camera. However, as this work is carried out on atwo-dimensional image, the problem mentioned above is not resolved: anautomatic use system cannot determine the distance of the source, and ifit is indeed at the position of a detail of the environment on which itis superimposed, or if it is situated in front of or behind it on theradius of sight of the camera.

In addition, tomography methods are known which make it possible to knowthe positions of radioactive sources in a three-dimensional environment;the cameras (or, more generally, the means of taking two-dimensionalimages) are moved around the object and the information read on theimages is combined in order to derive therefrom the position of thesources. This amounts to inverting, directly or indirectly, a system ofequations expressing the fact that the radiation received by each imagepoint is the sum of the radiation emitted along the line of projectionor sight of the camera which ends at this point. However, it isnecessary to know the position of the camera each time an image istaken, which is not always possible in the situations envisaged here,since the robots are not always sufficiently precise, nor even providedwith position coders which indicate where they have arrived.

Finally, it is necessary to cite the international patent application WO96/20421, which describes a double method of tomography andsuperimposition of two three-dimensional images thus obtained, where oneillustrates the visible details of the object examined and the otherdepicts a view of the object by X-ray or the like. However, the twoimages are calculated in the same way and separately (except in order toapply correction calculations for the effects of the distortion,enlargement, etc produced by each of the photographic means); they areput in a relationship of equality and have the same importance.

This patent therefore does not give the idea of using a visual image ofthe environment in order to assist in determining the positions of pointradiation sources in this environment, without it being necessary tohave recourse to the conditions of obtaining tomographic images.

SUMMARY OF THE INVENTION

The object of the invention is therefore to make it possible tocompletely and precisely locate radiation sources, radioactive orothers, in a three-dimensional environment.

The essential idea of the invention is that a three-dimensional model ofthe environment is used, established in advance by taking visual images,on which there are placed the sources registered on other images, whichare correlated with the visual images.

The purpose of the model is therefore not only to provide a graphicalrepresentation of the positions of the sources in the environment, butparticularly to help to determine these positions.

In its most general form, the invention thus concerns a method for thethree-dimensional mapping of sources of radiation in an environment,comprising a first taking of a visual image of the environment and afirst taking of an image of the sources, characterised in that itcomprises a second taking of a visual image of the environment and asecond taking of an image of the sources; an establishment of a visualthree-dimensional model of the environment by searching for andidentification of analogous elements of the visual image and then bylocation calculations for the homologous elements of the visual images;a location in the model of the environment of projection lines leadingfrom the sources to the images of the sources; and calculations ofpositions of points of intersection of the projection lines in themodel.

A device for implementing this method comprises a device for takingimages of the radiation, a pair of means of taking visual images of anenvironment of the radiation sources, the means of taking visual imagesbeing oriented in directions such that they have all or part of theirfield of vision in common and mounted on a rigid common support which isnon-deformable but adjustable with the means of taking images of theradiation, and photogrammetry means able to reconstitute a visualthree-dimensional model of the environment from the visual images and tosuperimpose a three-dimensional model of the radiation sources usingimages of the radiation on the visual model.

BRIEF DESCRIPTION OF THE DRAWINGS

The commentary on concrete embodiments of the invention, given forpurposes of illustration and applied to the detection of gamma radiationsources, will now be developed by means of the following figures:

FIG. 1 is a general view of the mechanical means of the device,

FIGS. 2 and 3 are two details of the device,

FIG. 4 illustrates the mapping method,

FIG. 5 illustrates the formation of the images,

FIG. 6 depicts a calibration sight,

FIG. 7 shows the geometric definition of certain calibration parametersfor the device,

FIGS. 8 and 9 are two flow diagrams setting out the methods ofcalibrating and servicing the device, and

FIG. 10 is a diagram of the means, notably computer means, serving thedevice.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is now made to the figures. The first shows the generalappearance of the device of the invention: a frame 1 comprises a centralhousing 2 from which there start two lateral arms 3 pointing in oppositedirections and an upper appendage 4. The central housing 2 contains analveolus, open towards the front, intended to house a gamma camera 6.Video cameras 7 are mounted at the ends of the lateral arms 3. Finally,a spotlight 8 is mounted at the top of the upper appendage 4.

The video cameras 7 are screwed onto rotating plates 9, each terminating(see FIG. 2) in a bottom journal 10 engaged in a cup 11 in a ring 12welded to the end of the associated arm 3. The ring 12 receives alateral locking screw 13, the end of which emerges in the cup 11 andclamps the lower journal 10, holding it in place at the requiredorientation. Finally, a fixing screw 14 is engaged in a thread 15 in theplate 9 coaxial with the lower journal 10, and this screw 14 passesthrough the lateral arm 3 and the ring 12 and holds the assemblyclamped. This arrangement makes it possible, as will be seenimmediately, to orient the video cameras 7 as required in the same planewith respect to azimuth in order to give them the desired angle ofconvergence.

The central housing 2 essentially comprises (see FIGS. 1 and 3) twoopposite lateral walls 16 provided with a recess 17 for receiving ajournal 18 supporting the gamma camera 6. The screws 19 are engagedthrough the lateral walls 16 and journals 18 and screwed into the gammacamera 6 in order to clamp it in the required position: it is in factpossible to cause it to rotate about a horizontal axis defined by thejournals 18 when the screws 19 are loosened and therefore to adjust itsorientation with respect to elevation. The frame 1 is mounted on a base20 which can be designed to rotate and which is fixed to a support, notshown, robot arm or the like according to the application.

The method of locating radioactive sources can be described fairlysimply by means of FIG. 4: the device, brought to a suitable distancefrom the environment to be explored, takes a first series of imagesthereof by means of the video cameras 7 and gamma camera 6 withoutmoving. The two visual images 25 and 26 of the environment thus obtaineddepict substantially the same subject at different angles of view usingthe video cameras 7. An automatic comparison of these visual images 2526 consisting of identifying and comparing the homologous points of theimages, representing the same noteworthy detail of the environment,makes it possible to deduce the position of the points in theenvironment with respect to the device, including their distance. Theenvironment can therefore be modelled in three dimensions and theposition of the device in the environment determined. Seeking thehomologous points on the two visual images 25 and 26 is carried out byone of the specialist software packets which are now availablecommercially.

There are even software packages capable of directly distinguishingcomplete details on an image, by recognition of shape associated with acorrelation of images, and finding them on an analogous image.

When a pair of homologous points is identified on the visual images 25and 26, two lines are derived therefrom, ending at the video cameras 7and through which the real point in the environment is projected ontothe visual images 25 and 26. The separation and angle of convergence ofthe video cameras 7 being known, an elementary triangulation calculationgives the position of intersection of the projection lines, that is tosay the position of the point in the environment.

An image 27 of radioactive sources is taken by the gamma camera 6.However, a complete determination of the position of the sources makesit necessary to take another image of sources 28, which is obtainedafter having moved the device whilst causing it to aim at the sameenvironment, and to compare these two images of sources 27 and 28 inorder to assess the difference between the sources and the gamma camera6.

A more detailed description of the method will now be given.

The work of seeking the position of the points in the visual radioactiveenvironment from the images 25 to 28 is effected by photogrammetrysoftware in conjunction with triangulation calculations, but apreliminary calibration must be undertaken in order to determine theexternal and internal parameters of the cameras 6 and 7, that is to saytheir relative positions and orientations on the one hand and theircharacteristics of restoration of the environment on the images whichthey produce, on the other hand.

It is first of all necessary to know the internal parameters. The gammacamera 6 and the video camera 7 can be represented (see FIG. 5) by animage plane 30, on which the image 25, 26, 27 or 28 is taken, and anoptical centre P in front of this image plane 30, through which therepass all the radii 32 which impinge on the image plane 30. The opticalcentre is formed by the diaphragm for the video cameras 7 and by acollimator 33, preceding a pinhole enclosure 34, at the end of whichthere is situated the image plane 30 for the gamma camera 6.

An object 35 at which a camera is aimed appears on its image plane inthe form of an object image 36, which may be affected by distortions,but if the object 35 is a sight of a known form and the position of thecamera 6 or 7, and in particular of the image plane 30 and of theoptical centre P, is calculated with respect to the object 35, it ispossible to establish a correspondence between each point 37 of theobject 35 and the point 38 by means of which it is represented on theimage plane 30, by determining the radius 32 which connects them, andwhich passes through the optical centre P; this radius 32 can becompletely defined by the coordinates of the optical centre and thepoint 37. The calibration consists precisely of drawing up a look-uptable between each point 38 of the image taken by the camera 6 or 7 andthe direction of sight (the radius 32) associated with this point andpassing through the optical centre P. This table is immutable for givencamera settings and serves to determine the directions of the points ofunknown objects whose images are then taken. The object 35 of knownshape can be the sight illustrated in FIG. 6, composed of a lattice ofbars which intersect but which are rather separated from each other andcarry points 39 which are identifiable with precision and which enablethe photogrammetric software to easily find the noteworthy points in theimage and to identify them with respective points on the sight.

It will then be understood, returning to FIG. 4, that any point 40 on anobject 41 in an environment which is unknown at the start and aimed atby the two video cameras 7 will appear on the visual images 25 and 26with the appearance of two points 42 and 43 whose positions on theseimages will make it possible to derive the directions, on the radii 44and 45, from the optical centres (denoted here P1 and P2) of the twovideo cameras 7. The intersection of the radii 44 and 45 is thencalculated, which gives the position of the point 40 with respect to thevideo cameras 7. The generalisation of this method to all the pairs ofhomologous points on the visual images 25 and 26 makes it possible tofind the shape and position of the object 41 in three-dimensional space.

If a point 100 is defined as a source, this appears also on the sourceimage 27 of the gamma camera 6 with the appearance of a point 106. Theposition of the cameras 6 and 7 with respect to the object makes itpossible per se to know the direction of the point 100, which issituated on a radius 107. However, it is not possible to state withcertainty that the origin of the point 106 is indeed the point 100rather than another point on the radius 107. This is why the secondsource image 28 must be taken in order to give a second radius 108leading from the new position of the optical centre (P3) of the gammacamera 6 to the point 100, which can then be identified as the source bycalculating the intersection of the radii 107 and 108. It is alsonecessary to know the distance 109 between the two successive positionsof the optical centre P3 of the gamma camera 6, that is to say the baseof the triangulation undertaken on the radii 107 and 108, in order todetermine the position of the point 100 which is their intersection, andthe angle of rotation of the device between the two shots. These twoitems of information can be obtained by a direct measurement of themovements of the device if it is carried by a robot arm whosearticulations are provided with movement coders; otherwise, anexploration of the environment and of the object 101 can be recommencedusing new visual images 49 and 50 taken by the video cameras 7 at thesecond position of the device, in order to calculate the latter withrespect to the position of the object 41.

The method then comprises a synthesis of the two models of the object101, which can also be undertaken by the software for seeking andidentifying noteworthy homologous points of these models, in order toevaluate their differences in location and orientation compared with thevideo cameras 7 between the two shooting phases. When this evaluation iscompleted, it is possible to calculate the positions of the twoprojection lines 107 and 108 of the source in one of the models, andtherefore the position of their point of intersection 100 in this model.

Another variant of the method consists of exploiting a portion of thegamma image 28 which represents the environment 41: the gamma images 28obtained by the usual gamma cameras are in fact sensitive also tovisible light, which means that an image of the environment 41 issuperimposed on the image of the sources. This image of the environmentis often too tenuous and fuzzy to provide a suitable model of theenvironment 41 (although exceptions may be envisaged, so that theinvention could then be applied with a single camera, the visual imagesall being able to be superimposed on gamma images), but it can becorrelated with the model already obtained by the video cameras 7, bymeans of the software already mentioned, in order to calculate here alsothe positions of the lines 107 and 108 of projection of the source inthe model.

The method can, in general terms, be implemented with a single visualcamera 7, provided that there is available a measurement of thepositions of a shot in order to serve as a basis for the triangulationsfor constructing the model of the visual environment, or otherinformation, a few examples of which are given below.

It is however preferred to apply the invention with the device describedin detail, which comprises three cameras in total, since the model ofthe environment can be constructed much more quickly, without moving thedevice between two shots and using more effective software forperforming the triangulation calculations.

It makes it possible to work “in real time” if for example it is mountedon a robot arm having to manipulate the sources: the position of thesources is then calculated whilst the arm advances, and the two shootingtimes correspond to two phases of the advance of the arm. There is alsothe certainty that the two visual images 25, distant from a known base,will be similar and that seeking homologous points will almost always befruitful.

The rules for determining the external parameters of the device will nowbe given; this determination is made before using the device accordingto FIG. 4 and immediately follows the determination, explained above, ofthe internal parameters of the device; it therefore constitutes a secondpart of the calibration of the device.

The modelling of the object 40, effected on the visual images 25 and 26,also required using a triangulation and therefore knowing atriangulation base, which corresponded to the distance 51 between thevideo cameras 7. One variant consists of using multiple positions of thedevice coupled with known length information issuing from a plane or areference placed in the scene on the one hand, and calculation softwarefor adjusting the beam on the other hand. It was also necessary to knowthe position of the optical centre P3 of the gamma camera 6 in order todetermine the position of the radius 47. These external parameters ofthe device can be expressed by a file of the coordinates of six points,as shown by FIG. 7: the three optical centres P1, P2 and P3 of thecameras 6 and 7 and three points aligned with these respective opticalcentres on the central axes of site of the cameras 6 and 7; the latterpoints are numbered P4, P5 and P6 and can be situated at any distances,identical or not, from the points P1 to P3 with which they arerespectively associated. In addition there is no constraint on therelative orientations and positions of the central axes of sight, whichcan intersect or not, although the axes of the video cameras 7 aresupposed to intersect in the embodiment actually proposed; it is thuspossible to adjust, without any particular constraint, the positions ofthe cameras 6 and 7 on the lateral arms 3 and frame 1. The externalparameters of the cameras can therefore be summarised in the followingtable:

x(P1) y(P1) z(P1) x(P2) y(P2) z(P2) x(P3) y(P3) z(P3) x(P4) y(P4) z(P4)x(P5) y(P5) z(P5) x(P6) y(P6) z(P6)

where x, y and z designate the Cartesian coordinates of the point Punder consideration. The reference frame for measuring the coordinatescan be chosen as required, for example with the origin P1, the axis xpassing through P2 and the central axis of sight P1P4 included in theplane of the axes x and y. Under these circumstances, it is possible towrite x(P1)=y(P1)=z(P1)=y(P2)=z(P2)=z(P4)=0. These seven constraints fixthe seven degrees of freedom of the device, that is to say 3 rotations,three translations and one distance base. All these coordinates can becalculated by calibration triangulations undertaken by causing thedevice to turn about the sight in FIG. 6 and placing it at differentpositions in order to take series of shots: the distances of the opticalcentres P1, P2 and P3 of the cameras 6 and 7 to the sight are thenknown, and as soon as a noteworthy point on the sight is registered onthe images of the cameras 6 and 7, the directions of the radii whichconnect it to the optical centres P1, P2 and P3 are determined as afunction of the look-up table, and finally the relative positions of theoptical centres P1, P2 and P3, and then suitable positions for thepoints P4, P5 and P6. The calculations of the positions of the points P1to P6 are undertaken from several noteworthy positions of the sight inorder to have available more numerous data, whose average is finallytaken.

FIGS. 8 and 9 are flow diagrams which set out the steps undertaken inorder to calibrate the device and then to use it for effecting amapping. A complete diagram of the operating system of the device isfinally given in FIG. 10: there is found therein a first shooting module52 which is connected to the gamma camera 6 and which records itsimages; a second shooting module 53 connected to the video cameras 7 andwhich records their images; a module for seeking homologous points 54which is connected to the second shooting module 53 and which seeks thehomologous points, corresponding to one and the same point on the objectsighted, present on the images of the two video cameras 7; aphotogrammetry module 55 which establishes essentially the directions ofthe points of the object aimed at according to the positions of theimages of these points on the views; a modelling module 56 whichcalculates the positions of the points on the object with respect to thedevice; a display module 57; and a control module 58 responsible for therunning of the four previous modules, their relationships and thetriggering of the shots. All these modules can in reality be groupedtogether on the same computer and be embodied in particular by asoftware package.

What is claimed is:
 1. A method for mapping sources of radiation in athree-dimensional environment, comprising: providing an assemblycomprising a support, means for taking visual images, and means fortaking images of the sources, said means for taking visual images andsaid means for taking images of the sources being secured together tothe support; determining relationships between respective positions andorientations of said means for taking visual images and said means fortaking images of the sources; taking at least two visual images of theenvironment at different angles of view with said assembly displaced atone imaging position and at least two images of the sources with saidassembly displaced to at least two imaging positions; establishing athree-dimensional model of the environment from said at least two visualimages, identifying homologous elements in the at least two visualimages, and then computing locations of said homologous elements;determining lines of projection of the sources leading to the means fortaking images of the sources for the at least two imaging positions;locating said lines of projection in said three-dimensional model basedon said relationships; and computing positions of intersection points ofsaid lines of projection in said three-dimensional model, which arepositions of the sources.
 2. A method according to claim 1, wherein saidthree-dimensional model of the environment is established usingmeasurements of translation and rotation displacements of said assemblybetween said at least two imaging positions.
 3. A method according toclaim 1, further comprising: a preliminary calibration comprisingcorrecting any distortion in images by establishing a look-up tablebetween points on the images and directions of sight of a known objectat a given position of said means for taking visual images and saidmeans for taking images of the sources.
 4. A method according to claim1, further comprising: a preliminary calibration comprising assessingpositions of optical centers of said means for taking visual images andsaid means for taking images of the sources with respect to a knownobject and assessing directions of central axes of sight of said meansfor taking visual images and said means for taking images of thesources.
 5. A method for mapping sources of radiation in athree-dimensional environment, comprising: providing an assemblycomprising a support, two means for taking visual images, and one meansfor taking images of the sources, said two means for taking visualimages being oriented in directions such that they have at least a partof their field of vision in common, said two means for taking visualimages and said one means for taking images of the sources being securedtogether to the support; determining relationships between respectivepositions and orientations of said two means for taking visual imagesand said one means for taking images of the sources, and relationshipsbetween respective positions and orientations of said two means fortaking visual images; taking at least two visual images of theenvironment with each of said two means for taking visual images withsaid assembly displaced at least at two imaging positions and at leasttwo images of the sources with said assembly displaced to the respectivetwo imaging positions; establishing three-dimensional models of theenvironment from said at least two visual images for each one of saidtwo means for taking visual images, identifying homologous elements inthe visual images, and then computing locations of said homologouselements for each of said imaging positions; determining lines ofprojection of the sources leading to said one means for taking images ofthe sources for the respective two imaging positions; locating in eachthree-dimensional model said lines of projection determined for a sameimaging position by using said relationships; combining thethree-dimensional models into an overall model and locating said linesof projection in said overall model; and computing positions ofintersection points of said lines of projection in said overall model,which are positions of the sources.
 6. A method according to claim 5,wherein one of said two means for taking visual images is said means fortaking images of the sources.
 7. A method according to claim 5, whereinsaid overall model of the environment is established using measurementsof translation and rotation displacements of said assembly between saidimaging positions.
 8. A method according to claim 5, further comprising:a preliminary calibration comprising correcting any distortion in imagesby establishing a look-up table between points on the images anddirections of sight of a known object at a given position of said twomeans for taking visual images and said one means for taking images ofthe sources.
 9. A method according to claim 5, further comprising: apreliminary calibration comprising assessing positions of opticalcenters of said two means for taking visual images and said one meansfor taking images of the sources with respect to a known object andassessing directions of central axes of sight of said two means fortaking visual images and said one means for taking images of thesources.